statsmodels is being used by plotly express (for instance for px.scatter with linear regression trend, with parameter trendline="ols")

Example: I've develop a test application in Dash that you can see at

https://github.com/pdenapo/normal_bivariada

When I've tried to deploy it to heroku (following the instructions in the Dash documentation), my application
didn't work since the statsmodels was missing in the requirements.

I've fixed this by adding a

import statsmodels

statement to my code (event though this module is not used anywhere in my code), and regenerating the requirements.txt

I'm not submitting a pull request since I am not sure of which file in plotly source should be modified.